WASHINGTON (CNN) -- Sophisticated submarine-like boats are the latest tool drug runners are using to bring cocaine north from Colombia, U.S. officials say.

Semi-submersible boats used to smuggle drugs are gaining in quality, the Coast Guard says.

Although the vessels were once viewed as a quirky sideshow in the drug war, they are becoming faster, more seaworthy, and capable of carrying bigger loads of drugs than earlier models, according to those charged with catching them.

"They tend to be one of a kind," U.S. Coast Guard Commandant Thad Allen said. "They cost up to a million dollars to produce. Sometimes they are put together in pieces and then reassembled in other locations. They're very difficult to locate."

The boats are built in the Colombian jungle. They sail largely beneath the surface of the water but cannot submerge completely like a true submarine.

But they are the latest escalation of a tactical race between smugglers and the U.S. Coast Guard.

In the past three months the Coast Guard has learned of more semi-submersible vessels smuggling drugs than it did in the previous six years, when there were 23 cases, officials said. Watch the Coast Guard chase down a semi-sub »

U.S. Coast Guard intelligence officers predict 85 cases this year and 120 next year.

In some instances, the semi-subs are towed behind other vessels and are scuttled if they are detected, Allen said. Authorities are investigating reports that some semi-subs are unmanned and are operated remotely, he said.

Encounters have become so frequent -- and the dangers of boarding the vessels so pronounced -- that the Coast Guard is pushing for legislation that would make the use of "unflagged" semi-submersibles in international waters a crime punishable by up to 20 years in prison, even if authorities can't recover drug evidence because the smugglers scuttle the transports.

"There's really no legitimate use for a vessel like this," Allen said.

An unflagged vessel is one not registered with a government.

Allen believes the semi-subs are a response to the Coast Guard's tactic of using snipers in helicopters to shoot out engines on smugglers' speedboats. The submersibles' engines are beneath water level.

"We're seeing an evolution in the construction," he said. "Early on we saw fiberglass and now we're seeing steel."

Early semi-subs were capable of carrying 4 or 5 metric tons of cargo; newer ones can carry 12 metric tons, Allen said. Their speed has increased to 12 knots, which is "a pretty good speed on the ocean."

Despite the increase in the use of the semi-subs, Drug Enforcement Administration officials say most drugs still are transported by traditional methods -- fishing boats, speedboats and airplanes.

But Frankie Shroyer, deputy chief of the DEA's Office of Enforcement Operations, called the use of semi-subs "an emerging threat and we are attacking it through our investigations and working with the interagency community."

The DEA's main focus, however, "is to dismantle entire organizations," he said. "So we are looking at the organizations that are building these things. ... These are the same organizations that are using containers, the same organizations that are using airplanes, same organizations using go-fast boats."

Allen said the Coast Guard, the Department of Defense and others are working on how to board the vessels. "In many cases, they don't stop. And it's difficult to slow them down," he said.

The Coast Guard says drug runners also are resorting to putting refueling vessels far offshore so drug-carrying boats can avoid coastal areas, and even liquefying cocaine and concealing it in fuel.

The semi-subs are "another adaptation ... that we're going to have to adapt to ourselves," Allen said.

Last year, the Coast Guard seized a record 355,000 pounds of cocaine worth an estimated $4.7 billion -- a 2 percent increase over 2006.

The Coast Guard's largest cocaine bust ever came in 2007 -- 42,845 pounds stacked in large bundles on the deck of a freighter off the coast of Panama.E-mail to a friend